Transistor current control circuit



P 1965 J. G. COTTEN ETAL 3,181,010

7 TRANSISTOR CURRENT CONTROL CIRCUIT Filed Dec. 28, 1962 5%: 6528 8 ll ll.

INVENTORS JAMES G. COTTEN JAMES L. THOMAS ATTORNEY womnom od 3 181 010 VTRANSESTOR QURlENT CGNTRGL CIRQUET James G. tCotten, Anaheim, and JamesL. Thomas, La Hahra, Caiizh, assignors to North American Aviation,

Inc.

Filed Dec. 28, 1962, Ser. No. 247,964 12 Claims. (Cl. 367-885) Thisinvention pertains to an improved transistor circuit for controllingload current, and more particularly to a control circuit of the typeconnected in series between a source of voltage and a load. I

Series connected circuits for regulating or'controlling current througha load have been devised in the past with, for. example, vacuum tubeamplifiers, magnetic amplifiers andtransistor amplifiers. Vacuum tubeamplifiers are still widely used, particularly for high voltages but arevibration sensitive and are therefore unsuitable for many applicationsespeciallywhere space and weight must be conserved. Magnetic amplifiersare noisy and therefore also unsuitable for many applications, againespecially where space and weight must be conserved. Transistoramplifiers are not vibration sensitive or noisy and are thereforeideally suited for use where space and weight must be conserved, butthey are generally limited in their use because of their limited voltagerange set by the breakdown voltage of their reverse biased collectorjunctions.

To overcome the limited voltage range of available power transistors, itis customary to place a plurality of transistors in series between thesource and the load in order to divide among them the difierence betweenthe voltage of the current source and the voltage across the load. Insuch a circuit a voltage dividing network is employed to bias-thetransistors for substantially equal conduction. That "is accomplished byconnecting the voltage dividing network in parallel with theseries-connected transistors and connecting the base electrodes of eachtransistor, except the base electrode of the one used as the seriescontrolling element, to equal divisions of the dividing uetworkpHowever, this biasing technique does not assure that theseries-connected transistors will share the voltage drop across themequally with the one used for control since the one used for control isindependently biased.

An object of this invention is to provide an improved control circuit inwhich current to a load is controlled by a circuit including a pluralityof transistors connected in series with the source and the load.

In accordance with the present invention, a plurality of transistors areconnected in emitter-to-colle ctor series for the conduction ofcontrolled current to the load and in parallel with a voltage dividingnetwork employed in a novel manner to assure that each series-connectedtransistor shares equally the difference in voltage between the sourceof current and the load. The first transistor in series has its baseelectrode connected to a control input terminal adapted to receivecontrol signals for modifying base current thereby controlling currentthrough the load. The base electrodes of the successive series-connectedtransistors are connected to successive points along the voltagedividing network by the base-to ice to the drawing in which the solefigure shows a circuit diagram of an illustrative embodiment.

The positive terminal of a D.C. source 10 is directly connected to aload 11. A return path from the load 11 to the negative terminal of theDC. source It) is provided by a plurality of series-connectedtransistors Q Q Q Q and Q The baseelectrode or" the first transistor Qis connected to a' control input terminal 12 which is adapted to receivecontrol signals that control the base.

current therein to vary its emitter current and thereby the controlcurrent through the load 11.

The difference between the voltage across the load 11 and the voltage ofthe source 10 is impressed across the series-connected transistors Q Q QQ and Q and a voltage dividing network comprising resistors 13, 14, 15,16 and 17 which divide that voltage difference proportional to theirresistance. The voltage dividing network presents relatively high sourceimpedance voltage references for transistors Q Q Q and Q The transistorQ3 which has its base electrode connected to the first voltage dividingpoint at the junction between the resistors 13 and 14, couples thecollector of the control transistor Q to the base electrode of thesuccessive series-connected transistor Q The transistors Q Q and Qsimilarly couple the collector electrodes of the series-connectedtransistors Q Q and Q, to the base electrodes of the respectivelysuccessive transistors Q Q6 and Q8- Due to the high gain of transistor Qthe'resistor 13 which provides a voltage reference for the baseelectrode of transistor Q appears as a low impedance to the base of thetransistor Q and as a still lower impedance to the collector of thetransistor Q thus clamping the emitterto-collector circuit of thetransistor Q to the voltage across the resistor 13. Theemitter-to-collector circuits of the transistors Q Q Q and Q; aresimilarly clamped to the voltages acrossthe respective resistors 14, 15,16 and 17.

The control transistor Q is biased for class A operation by a DC.voltage source 18 of about -l2 volts through a resistor 19. Theremaining transistors Q Q Q and Q; are similarly biased for class Aoperation by a DC voltage source 20 of about 6 v. through respectiveresistors 21, 22, 23 and 24.

To decrease the current through the load 11, base current in the controltransistor Q is decreased by a control signal applied to the inputterminals 12, thereby reducing emitter junctions of a plurality ofcomplementary type; V transistors, the collector electrodes of which areeach connected to the collector electrode of their respectively pro-'the emitter current of the transistor Q and causing a decrease incurrent through the load 11. Thus, by varying the base current of thecontrol transistor Q, the voltage across the dividing network ofresistors 13 to 17 is changed. That change in voltage is divided by theresistors 13, 14, 15, 16 and 17, causing each of the transistors Q Q Qand Q; to continue to share with the transistor Q the voltage differencebetween the source 18 and the load 11. That is accomplishedautomatically by a decrease in current through the load 11 which causesan increase in voltage across the resistors 13 to 17 in the.

dividing network thereby causing an increase in base currents throughthe transistors Q Q Q and Q which in turn cause a decrease in basecurrents through the transistors Q Q Q and Q To assure equal division ofthe difierence in voltage between the source It and the load 11 by thetransistors Q Q Q Q and Q the resistors 13 to 17 of the dividing networkare made substantially equal, such as 5K ohms. For more equal divisionof the voltage difference between the series-connected transistors, eachof the resistors 13, 14, 15, 16 and 17 should be made progressivelylargerv due to the leakage of return current from the load to the sourceit) through the base-to-emitter junctions of the transistors, Q Q Q andQ Since the base-to- 3. emitter current for each of the transistors Q QQ and Q would be substantially the same, the exact, values selected forthe resistors 13 to 17 may be 4.5K, 4.7 5K, 5K, 5 25K and 5 .SK ohms,respectively. 'However, for many 7 applications not requiring too largea variation in load cur rent, the resistors may be 4.5K, 5K, 5K, SKand5.5K

ohms, respectively, for a good approximation of equal voltage sharingbetween the transistors Q Q Q Q and Q 'The biasing resistors 21 to 24are selected tobe progressively smaller to assure sufiicient forwardbias current through the emitter-to-base junctions of the respectivetransistors Q Q Q and Q; to keep them biased for class A operation. Thatwould require thatat least diodes of the type lN647 are employed foremitter bias of each transistor in place of one of the diodes shown,

' such as diode D the resistors 21, 22, 23 and 24 should be selected tobe approximately 25.414, 239K, 22.5K and 21K ohms. This requirement isdue inpart to the fact that each of the successive transistors Q Q Q andQ; has progressively more emitter biasing diodes in its emitter-to-basecurrent path from the source 20.

The coupling diodes D to D between stages also provide the emitter biasnecessary to operate the transistors Q Q Q -and Q For example, thetransistor Q, has its collector connected toits emitter through thediode D and the emitter-to-base junction of the transistor Q The voltagedrop through the'emitter-to-basejunction of the transistor. Q normallywill not provide a potential on the collector of'the' transistor Q whichis suffieiently positive with respect to the' emitter for operation. Toprovide sufiicient voltage, the impedance of the diode D is added inseries with the impedance of the emitter-tobase junction in'thetransistor Q If necessary, a second coupling diode maybe connectedinseries with the diode D depending on the types of transistors anddiodes being employed. For example, if the transistor Q is of the type2Nl556A, two diodes of the type 1N647 should be connected in seriesbetween the collector of 'thetransistor Q and the emitter of thetransistor Q Y Capacitors 31 to 35 are connected to the base electrodesof the transistors Q Q Q and Q in order to stabilize the operation ofthe circuit at high frequencies; For that purpose, relatively largecapacitors are selected such .as capacitors having a capacitance ofabout .1 microfarad.

A resistor 40 is connected between the emitter of the transistor and itscollector in order to provide sufiicient current forthe transistors Q QQ Q and Q if the control transistor Q isv permitted to be driven tocutofi by a large positive signal at the input terminals 12'. Theminimum current which the resistor 49 should provide is the sum of thecollector cutoif current 1 of the transistor Q the base currents-l ofthe transistors Q Q4,

Q and Q 'in addition to the collector current I lOf the transistor QUsing the components suggested hereinbetore in the illustratedembodiment, the resistor 44) should be selected to be 125K ohms. Y

The illustrative embodiment of the invention is in tended for use with aDC. source which provides a voltage that may vary to as high as about275 volts in order to regulate the load voltage at 155 volts.Accordingly, the series-connected transistors are expected to divideequally between them a voltage difierence as high as 120 volts. If ahigher difference between the voltages of the DC. source and the load isto be divided, additional stages comprising complementary pairs oftransistors,

' without departing from those principles. The appended. claims aretherefore intended to embrace any such mod-. ifications.

suchas transistors Q and Q may be connected in series.

For automatic regulation of load voltageby controlling load current, avoltage sensing circuit may be connected across the load to produce asignal proportional to the load voltage. That signal may then betranslated to the input terminals 12 as a control signal to cause aproportional change in the base current of the control transistor QFor-instance, a disturbance, such as a variation in the load 11 or theDC. source 16, that would tend to increase the load voltageqcould besensed to produce a proportional control signal that tends to decreasethe on-biasing' base current of the transistor Q thereby reducing itsemitter current proportionately and causing the load voltage tobeproportionately.decreased.

While the principles of the invention have now been. madeclear in anillustrativeembodiment, obvious modifi cations particularly adapted forspecific .applications, en-

vironments and operating requirements may be made What is claimed is:

l. ln an improved circuit for controlling current to a load c afirsttransistor of one conductivity type serially connected betweenavoltag'e source and said load, said.

first transistor having a base electrode, input electrode and outputelectrode,

rneans for controlling the base electrode-current of said.

source and load, said secondtransistor having a.

conductivity type conbase electrode, input electrode and outputelectrode,

means for biasing the base electrode-of said second transistor forconductionbelow saturation,

first and second resistors serially' connected between said voltagesource and said load, and a third transistor of another conductivitytype complementary to said first andsecond transistors;

said third transistor having a base electrode, input electrode andoutput electrode, and having its input 7 and output electrodes connectedin series between the output electrode of said first transistor and thebase electrode of said second'transistor, and having its base electrodeconnected to a junction between said first and second resistors.

2. The circuit as defined in claim 1 including impedance means seriallyconnected between the output electrode of said first transistor and theinput elect-rode ofsaid second transistor for biasing said input andoutput electrodes of said third transistor for operation.

3. The 'circuit as defined in claim 2 wherein said impedance meanscomprises a semiconductor diode.

4. The circuit as defined in claim 2 wherein said first and secondresistors, associated with said first and second transistors in aparallel relationship between said'source a and said load, aresufliciently unequal in resistance to render the voltage drop acrossthem substantially equal, thereby compensating for the current throughthe base electrode of said thirdtransistor which flows through only'transistor being of a first conductivity type and hav-.

ing base, emitter and collector electrodes the emitter 3,181,010 r s a aplurality of semiconductor diodes, each serially conelectrodes connectedin series with said: source and said nected between the collectorelectrode of one of said load, and having a base electrode for;controlling the curfirst plurality of transistors and the emitterelectrode rent to the load comprising of an adjacent one of said firstplurality of trana first plurality of series-connected transistors ofsaid sistors connected in series for biasing the emitter 5 firstconductivity type connected'in series with said electrode of anassociated one of said second plusource and said control transistorforsharing the rality of transistors with respect to its collector elecdiiference between the voltages of said source and trode operation. ofsaid load, each transistor of said given conduc- 11. An improved circuitfor controlling current to a tivity type having a base, a collector andan emitter load comprising 1O electrode, 7 I

a voltage source, means connected'to the base electrodes of said first aload connected in series with said voltage source, plurality oftransistors for providing bias current a first return path from saidload to said voltage source for class Aoperation, v

through a first plurality of series-connected trana plurality ofresistors, one resistor associated with sistors of a first conductivitytype, 'eachtransistor each of said first plurality of transistors and anaddihaving a base electrode biased for operation as a tional oneassociated with said control transistor, class A amplifier and having aninputand output said resistors being connected in series'with saidelectrode, source and said load for dividing the voltage ap a pluralityof diodes coupling the output electrode of proximately equally betweenthem,

each of said first plurality of transistors to the input a secondplurality oftransistors of a conductivity type electrode of the nextseries-connected transistor, complementary to said given conductivitytype, one means for applying a control signal'tothe base electransistorassociated Witheach of said first plurality trode of one of said firstplurality of transistors, of transistors, each of said second pluralityof trana second plurality of transistors of a second conducsistorshaving a base electrodeconnected to a'junctivity type complementary tosaid first conductivity tion between two of said Vseries=connectedresistors,

type equal in number to one less than the number an emitter electrodeconnected, to the base electrode of said first plurality of transistors,and connected of its associated one of said first plurality of trantoprovide a controlled current path between the sistors and its collectorelectrode connected to the output electrode of one of said firstplurality of serie collector electrode of the preceding seriesconnectedconnected transistors and the base electrode of the 39 tr i t th lle toel t d v f th 1 m h. next of said first plurality of transistorsinseries, and menfary ptype transistor associated h h fi a voltagedividing network connected in parall l With series-connected transistorhaving its collectortelec- Said first return P for Said load current,the base trode similarly connected to the collector electrode electrodesof said second plurality of transistors bef id o t l t i t a ingCODIIEC'EEd'IO intermediate points along said voltd means f apglying aontrol ignal t th b f age dividing network to control current flowthrough id t l t i t I said second plurality of transistors thereby tovary the base current through said first plurality of tran- RetereneesCited by the Examiner; sistorsv and effectively clamp said firstplurality of 40 UNET STATES PATENTS transistors to voltages betweentheir input and output terminals as determined by said voltage dividingnetwork r 3,056,043 9/62 Baude 30788.5 12. An improved circuit forcontrolling a current 3,109,940 11/63 Baude from a source to a loadthrough a control transistor of 5 t a given conductivity type havingcollector and emitter ARTHUR GAUSS, Primary Examiner.

1. IN AN IMPROVED CIRCUIT FOR CONTROLLING CURRENT TO A LOAD A FIRSTTRANSISTOR OF ONE CONDUCTIVITY TYPE SERIALLY CONNECTED BETWEEN A VOLTAGESOURCE AND SAID LOAD, SAID FIRST TRANSISTOR HAVING A BASE ELECTRODE,INPUT ELECTRODE AND OUTPUT ELECTRODE, MEANS FOR CONTROLLING THE BASEELECTRODE CURRENT OF SAID FIRST TRANSISTOR, A SECOND TRANSISTOR OF SAIDONE CONDUCTIVITY TYPE CONNECTED IN SERIES WITH SAID FIRST TRANSISTOR,VOLTAGE SOURCE AND LOAD, SAID SECOND TRANSISTOR HAVING A BASE ELECTRODE,INPUT ELECTRODE AND OUTPUT ELECTRODE, MEANS FOR BIASING THE BASEELECTRODE OF SAID SECOND TRANSISTOR FOR CONDUCTING BELOW SATURATION,FIRST AND SECOND RESISTORS SERIALLY CONNECTED BETWEEN SAID VOLTAGESOURCE AND SAID LOAD, AND A THIRD TRANSISTOR OF ANOTHER CONDUCTIVITYTYPE COMPLEMENTARY TO SAID FIRST AND SECOND TRANSISTORS,